case class encoder/decoder derivation bugfix

this fixes the implementation of 0.(5,6).12 for CCCassFormatEncoder[MyType] by adding a .derive function to supplement it. To take advantage of it, declare a case class and place the cached implicit in the companion object:

case class MyTable(s: String, i: Int, l: Option[Long])
object MyTable {
  implicit val encoder: CCCassFormatEncoder[MyTable] = CCCassFormatEncoder.derive
  implicit val decoder: CCCassFormatDecoder[MyTable] = CCCassFormatDecoder.derive
}